FIG. 1A is a block diagram of an RF transmitter circuit including a transmission line directional coupler. FIG. 1B is an ideal equivalent circuit diagram of the directional coupler. FIG. 1C is a diagram showing directions of signals at a coupling port and an isolation port of the directional coupler shown in FIG. 1B. FIG. 2A is a diagram showing an output signal from the coupling port of the directional coupler in polar coordinate form. FIG. 2B is a diagram showing an output signal from the isolation port of the directional coupler in polar coordinate form. FIG. 2C is a diagram showing both of the output signals from the coupling port and the isolation port of the directional coupler in polar coordinate form.
In the related art, a directional coupler is used for usage applications such as monitoring the power of a radio-frequency signal, monitoring and stabilizing a radio-frequency signal source, and measuring transmission and reflection of a radio-frequency signal. For example, a configuration is known in which, as shown in FIG. 1A, in the RF transmitter circuit 10 for a cellular phone device or the like, an automatic gain control circuit 14 detects output power from a directional coupler 11 and controls a transmission power amplifier 13 in accordance with the detection value to make minimum necessary power of the input power from the transmission power amplifier 13 to the directional coupler 11.
A transmission line directional coupler utilizes electric field coupling and magnetic field coupling between a main line and a coupling line (sub line). As shown in FIG. 1B, when a signal S1 is inputted into a signal input port (RFin) 23, the signal S1 is outputted from a signal output port (RFout) 24 via a main line 21. At that time, the main line 21 and the coupling line 22 are coupled to each other with electric field coupling by a distributed capacitance C between both lines and are coupled to each other with magnetic field coupling by a mutual inductance M. In the coupling line 22, due to the electric field coupling, a signal S2 propagates in the direction to a coupling port (hereinafter, referred to as CPL port) 25 and a signal S3 propagates in a direction toward an isolation port (hereinafter, referred to as ISO port) 26. In addition, in the coupling line 22, due to the magnetic field coupling, a signal S4 and a signal S5 propagate in a direction toward the CPL port 25 in a closed loop including a ground (GND).
As shown in FIG. 1C, in the CPL port 25, the phase of the signal S2 is +90° with respect to the phase of the signal S1. Meanwhile, the phase of the signal S4 in the CPL port 25 is +90° with respect to the phase of the signal S1 due to a phase delay (−90°) and the direction of the loop (+180°). In other words, the phases of the signal S2 and the signal S4 coincide with each other, and thus a signal obtained by combining the powers of the two signals is outputted from the CPL port 25. When the signal S2 and the signal S4 are shown in polar coordinate form, they are as shown in FIG. 2A.
As shown in FIG. 1C, the phase of the signal S3 at the ISO port 26 is +90° with respect to the phase of the signal S1. Meanwhile, the phase of the signal S5 at the ISO port 26 is −90° with respect to the phase of the signal S1. In other words, the signal S3 and the signal S5 are opposite to each other in phase, and thus the two signals cancel each other, and no signal is outputted from the ISO port 26. When the signal S3 and the signal S5 are shown in polar coordinate form, they are as shown in FIG. 2B.
When the output signal (S2+S4) from the CPL port 25 shown in FIG. 2A and the output signal (S3+S5) from the ISO port 26 shown in FIG. 2B are collectively shown in polar coordinate form, they are as shown in FIG. 2C. In other words, the isolation (the output from the ISO port) is zero, and a high directivity (ratio of a coupling amount and isolation) is obtained.
It is noted that such a characteristic can be realized by adjusting the distributed capacitance and the mutual inductance M between the main line 21 and the coupling line 22. In addition, the directional coupler shown in FIG. 1B has an ideal equivalent circuit, and the coupling coefficient K for the mutual inductance M between the main line 21 and the coupling line 22 is 1.
FIG. 3A is a circuit diagram of a directional coupler (an attenuator composite coupler) which is described in Japanese Unexamined Patent Application Publication No. 2009-044303 (Patent Document 1) and in which attenuators are provided at both ends of a coupling line. FIG. 3B is an ideal equivalent circuit diagram of the attenuator composite coupler. FIG. 4A is a frequency characteristic diagram of the attenuator composite coupler shown in FIG. 3B. FIG. 4B is a diagram showing, in polar coordinate form, frequency characteristics of a coupling amount and an isolation of the attenuator composite coupler shown in FIG. 3B.
In the related art, a directional coupler (attenuator composite coupler) 30 is proposed in which, in order to suppress characteristic variation caused by a load connected to the outside, attenuators 31 and 32 are provided at both ends of a coupling line, namely, between a CPL port 35 and a coupling line 22 and between an ISO port 36 and the coupling line 22 as shown in FIG. 3A (see Patent Document 1). The equivalent circuit of the attenuator composite coupler 30 is as shown in FIG. 3B. The attenuator composite coupler 30 has frequency characteristics as shown in FIG. 4A. It is noted that in this figure, isolation is indicated as IS, reflection loss is indicated as RL, coupling amount is indicated as CP, insertion loss is indicated as IL, and directivity is indicated as D. In addition, as shown in FIG. 4B, the isolation (IS) is nearly zero and a high directivity is obtained.